Optically variable pigments have been described in the patent literature since the 1960s. Hanke in U.S. Pat. No. 3,438,796 describes the pigment as being xe2x80x9cthin, adherent, translucent, light transmitting films or layers of metallic aluminum, each separated by a thin, translucent film of silica, which are successively deposited under controlled conditions in controlled, selective thicknesses on central aluminum film or substratexe2x80x9d. These materials are recognized as providing unique color travel and optical color effects.
The prior art approaches to optically variable pigments have generally adopted one of two techniques. In the first, a stack of layers is provided on a temporary substrate which is often a flexible web. The layers are generally made up of aluminum and MgF2. The stack of film is separated from the substrate and subdivided through powder processing into appropriately dimensioned particles. The pigments are produced by physical techniques such as physical vapor deposition onto the substrate, separation from the substrate and subsequent comminution. In the pigments obtained in this way, the central layer and all other layers in the stack are not completely enclosed by the other layers. The layered structure is visible at the faces formed by the process of comminution.
In the other approach, a platelet shaped opaque metallic substrate is coated or encapsulated with successive layers of selectively absorbing metal oxides and non-selectively absorbing layers of carbon, metal and/or metal oxide. To obtain satisfactory materials using this approach, the layers are typically applied by chemical vapor deposition techniques in a fluidized bed. A major shortcoming of this technique is that fluidized bed processes are cumbersome and require substantial technical infrastructure for production. An additional limitation related to the substrates utilized is that traditional metal flakes usually have structural integrity problems, hydrogen outgassing problems and other pyrophoric concerns.
The prior art approaches suffer from additional disadvantages. For instance, certain metals or metal flake such as chromium and aluminum, specifically when they are used as outer layers may have perceived health and environmental impacts associated with their use. The minimization of their use in optical effect materials should be advantageous due to their perceived impact.
The present invention provides a color effect material comprising a platelet-shaped substrate encapsulated with (a) a first layer selected from the group consisting of copper, zinc, an alloy of copper, and an alloy of zinc, wherein said first layer is highly reflective to light directed thereon; and (b) a second layer encapsulating the first layer and providing a variable pathlength for light dependent on the angle of incidence of light impinging thereon in accordance with Snell""s Law; and (c) a selectively transparent third layer to light directed thereon.
It is an object of the present invention to provide novel color effect materials (CEMs) which can also be prepared in a reliable, reproducible and technically efficient manner. This object is achieved by a CEM comprising a platelet-shaped substrate coated with: (a) a first layer of copper, zinc, an alloy of copper, or an alloy of zinc which is highly reflective to light directed thereon; and (b) a second layer encapsulating the first layer in which the second layer consists of a low index of refraction material, typically a refractive index from 1.3 to 2.5 and more specifically between 1.4 and 2.0, that provides a variable path length for light dependent on the angle of incidence of light impinging thereon; and (c) a selectively transparent third layer to light directed thereon.
The degree of reflectivity for the first encapsulating layer should be from 100% to 5% reflectivity, whereas the selective transparency of the third encapsulating layer should be from 5% to 95% transmission. More specifically, one would prefer to have 50-100% reflectivity and 50-95% transparency for the first and third encapsulating layers, respectively. The degree of reflectivity and transparency for different layers can be determined by a variety of methods such as ASTM method E1347-97, E1348-90 (1996) or F1252-89 (1996).
The substrate can be mica, aluminum oxide, bismuth oxychloride, boron nitride, glass flake, iron oxide-coated mica (ICM), silicon dioxide, titanium dioxide-coated mica (TCM), copper flake, zinc flake, alloy of copper flake, alloy of zinc flake, or any encapsulatable smooth platelet. The first layer encapsulating the substrate can be copper, zinc, an alloy of copper or an alloy of zinc. Of course, when the substrate is copper flake, zinc flake, alloy of copper flake or alloy of zinc flake, there is no need for such a first layer since it would be part of the substrate. The second encapsulating layer can be silicon dioxide or magnesium fluoride. The third encapsulating layer can be a precious metal, i.e., silver, gold, platinum, palladium, rhodium, ruthenium, osmium and/or iridium or alloys thereof. Alternatively, the third layer can be copper, silicon, titanium dioxide, iron oxide, chromium oxide, a mixed metal oxide, aluminum, and zinc.
An advantage of the present invention is that one does not have to start with a traditional metal flake which may have structural integrity problems, hydrogen outgassing problems and a host of other perceived issues (pyrophoric and environmental concerns) typically associated with metal flakes. The brass alloy used in this invention is much more chemically stable than aluminum and is known to have long term weatherability stability. Brass is nearly chemically inert which allows great flexibility in the chemical systems employed in the manufacture of such effect materials and in their applications in end uses such as in paint and polymer systems. Another advantage over the prior art is that brass, as one of the reflecting layers used in this invention, is a good reflector of white light and at the same time provides an attractive bulk color. The same would be true for an aluminum-copper alloy. Such an alloy is advantageous due to its attractive bulk color effect, while maintaining high reflectivity. Additionally, both brass and copper coated substrates provide the decorative/functional attributes of brass and copper, however under more environmentally favorable terms due to the reduced metal concentration since the CEM""s of the present invention are not pure brass or copper, rather brass or copper coated inorganic substrates. In addition, one can produce the CEM""s where the outer encapsulating layers are not made of brass. Another advantage over the prior art is that silver, or other metals such as gold, platinum, palladium, rhodium, ruthenium, osmium and iridium, as the final (outer) encapsulating layer of the effect material will impart electrical conductivity to the pigment which may be desirable in some applications such as powder coatings.
A surprising aspect of the present invention is that cost effective composite materials are created with desirable optical effect properties.
Metal layers are preferably deposited by electroless deposition and the non-metal layers preferably by sol-gel deposition. An advantage of electroless deposition (Egypt. J. Anal. Chem., Vol. 3, 118-123 (1994)) is that it is a world wide established chemical technique, not requiring cumbersome and expensive infrastructure compared to other techniques. The electroless deposition technique also allows one to control the degree of reflectivity of light quite accurately and easily by varying the metal film thickness. Additionally, the known procedures are generalized procedures capable of being utilized for coating a variety of surfaces. Furthermore, an encapsulating layer of a metal or metal oxide can also be deposited onto any of the substrates by chemical vapor deposition from an appropriate precursor (The Chemistry of Metal CVD, edited by Toivo T. Kodas and Mark J. Hampden-Smith; VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1994, ISBN 3-527-29071-0).
For deposition of alloys, a unique method has been developed as described in U.S. Pat. No. 4,940,523 which outlines a xe2x80x9cprocess and apparatus for coating fine particles.xe2x80x9d In addition, the technique can be used to deposit pure metals such as chromium, platinum, gold and aluminum, or ceramics.
The products of the present invention are useful in automotive, cosmetic, industrial or any other application where metal flake or pearlescent pigments are traditionally used.
The size of the platelet-shaped substrate is not critical per se and can be adapted to the particular use. In general, the particles have average largest major dimensions of about 5-250 xcexcm, in particular 5-100 xcexcm. Their specific free surface area (BET) is in general from 0.2 to 25 m2/g.
The CEMs of the invention are notable for multiple encapsulation of the platelet-shaped substrate.
The first metallic encapsulating layer is highly reflective to light directed thereon. The thickness of the first layer is not critical so long as it is sufficient to make the layer highly reflective. If desirable, the thickness of the first layer can be varied to allow for selective transmission of light. The thickness of the first metallic layer may be 5 nm to 500 nm and preferably 25 nm to 100 nm for copper, zinc or alloys thereof. A metallic layer thickness out of the above mentioned ranges will typically be either completely opaque or allow for substantial transmission of light. In addition to its reflective properties, the metallic encapsulating layer may exhibit unique bulk color effects depending on the film thickness. For example, a brass coating thickness of  greater than 50 nm will begin to exhibit a metallic gold bulk color, while maintaining good reflectivity. The mass percent of the coating will be directly related to the surface area of the particular substrate being utilized.
The second encapsulating layer must provide a variable pathlength for light dependent on the angle of incidence of light impinging thereon and therefore, any low index of refraction material that is visibly transparent may be utilized. Preferably, the second layer is selected from the group consisting of silicon dioxide (SiO2), suboxides of silicon dioxide (SiO0.25 to SiO1.95) or magnesium fluoride.
The thickness of the second layer varies depending on the degree of color travel desired. In addition, the second layer will have a variable thickness depending on a variety of factors, especially refractive index. Materials having a refractive index around 1.5 tend to require a film thickness of a few hundred nanometers for generation of unique color travel. For instance, a second layer has a preferable thickness of about 75 to 500 nm for silicon dioxide and for magnesium fluoride.
In one embodiment, the second layer is encapsulated by a selectively-transparent third layer that allows for partial reflection of light directed thereon. Preferably, the third encapsulating layer is selected from the group consisting of copper, silicon, titanium dioxide, iron oxide, chromium oxide, a mixed metal oxide, aluminum or alloys thereof. More preferably, the third encapsulating layer is one or more of the precious metals selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, osmium and/or iridium or alloys thereof.
Of course, the third layer can also contribute to the interference color of the pigment. Its thickness can vary but must always allow for partial transparency. For instance, a third layer has a preferable thickness of about 5 to 20 nm for silicon; about 2 to 15 nm for aluminum; about 2-10 nm for copper; about 2-10 nm for zinc; about 1-15 nm for titanium nitride; about 10 to 60 nm for iron oxide; about 10 to 60 nm for chromium oxide; about 10-100 nm for titanium dioxide; about 5 to 60 nm for a mixed metal oxide, about 5 to 20 nm for silver; about 3 to 20 nm for gold; about 3-20 nm for platinum; and about 5 to 20 nm for palladium. The precious metal and base metal alloys generally have a similar film thickness requirement compared to the pure metal. It is recognized that a film thickness out of the above range may be applicable depending on the desired effect.
All the encapsulating layers of the CEM of the invention are altogether notable for a uniform, homogeneous, film-like structure that results from the manner of preparation according to the invention.
In the novel process for preparing the coated platelet-like substrates, the individual coating steps are each effected by sputter deposition, electroless deposition or hydrolysis/condensation of suitable starting compounds in the presence of the substrate particles to be coated. Alloys, such as brass, can be deposited by a sputtering technique as described in U.S. Pat. No. 4,940,523. In addition, pure metals such as aluminum, copper and zinc, as well as others, can be sputter deposited. For instance, metals can be deposited from reduction of aqueous salts of the metals, such as HAuCl4, AgNO3, CuSO4, H2PtCl6, PdCl2. Silicon dioxide can be deposited from a compound selected from the group consisting of silicon tetraalkoxides such as tetraethoxysilane, bases such as sodium silicate and halide silanes such as silicon tetrachloride; titanium dioxide from tetraalkoxides such as titanium tetraethoxide, halide compounds such as titanium tetrachloride and sulfate compounds such as titanium sulfate, titanium nitride from titanium tetrachloride, tetrakis(diethylamido)titanium (TDEAT) and tetrakis(dimethylamido)titanium (TDMAT); iron oxide from iron carbonyl, iron sulfate and iron chloride; and chromium oxide from chromium carbonyl and chromium chloride.
In general, the synthesis of an alloy color effect material can be as follows: a platelet material such as glass flake is placed in an evacuated rotary cylinder as described in U.S. Pat. No. 4,940,523. A sputtering target of brass is utilized to coat the particulate material with a highly reflective coating. The highly reflective alloy coated substrate is removed from the evacuated cylinder and re-suspended in an alcoholic solvent such as butanol for deposition of the encapsulating silicon dioxide layer. A Stxc3x6ber process can be employed for the deposition of silicon dioxide on the metal coated mica or other substrate (C. Jeffery Brinker and George W. Schera, Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, Academic Press, Inc. (1990)). An alcoholic azeotropic mixture, such as ethanol and water, may be used in place of pure alcohol for the Stxc3x6ber process. The silica encapsulated metal coated platelet is filtered, washed and resuspended in a stirred aqueous medium. To the aqueous medium is added a silver precursor capable of depositing silver on the substrate by electroless deposition, along with a suitable reducing agent. The metal solution for electroless deposition is added as described above allowing for the deposition of a selectively transparent metal coating. The final particulate product is washed, dried and exhibits optical color effects as a function of viewing angle.
Depending on the thickness of the low refractive index second encapsulating layer, the final CEM will display multiple different color effects as a function of viewing angle (red, orange, green, violet). The platelet substrate acts as a carrier substrate. It may, or may not, have a contribution or effect on the final optical properties of the particulate.
The color effect materials (CEMs) of the invention are advantageous for many purposes, such as the coloring of paints, printing inks, plastics, glasses, ceramic products and decorative cosmetic preparations. Their special functional properties make them suitable for many other purposes. The CEMs, for example, could be used in electrically conductive or electromagnetically screening plastics, paints or coatings or in conductive polymers. The conductive functionality of the CEMs makes them of great utility for powder coating applications.
The above mentioned compositions in which the compositions of this invention are useful are well known to those of ordinary skill in the art. Examples include printing inks, nail enamels, lacquers, thermoplastic and thermosetting materials, natural resins and synthetic resins, polystyrene and its mixed polymers, polyolefins, in particular polyethylene and polypropylene, polyacrylic compounds, polyvinyl compounds, for example polyvinyl chloride and polyvinyl acetate, polyesters and rubber, and also filaments made of viscose and cellulose ethers, cellulose esters, polyamides, polyurethanes, polyesters, for example polyglycol terephthalates, and polyacrylonitrile.
Due to its good heat resistance, the pigment is particularly suitable for the pigmenting of plastics in the mass, such as, for example, of polystyrene and its mixed polymers, polyolefins, in particular polyethylene and polypropylene and the corresponding mixed polymers, polyvinyl chloride and polyesters in particular polyethylene glycol terephthalate and polybutylene terephthalate and the corresponding mixed condensation products based on polyesters.
For a well rounded introduction to a variety of pigment applications, see Temple C. Patton, editor, The Pigment Handbook, volume II, Applications and Markets, John Wiley and Sons, New York (1973). In addition, see for example, with regard to ink: R. H. Leach, editor, The Printing Ink Manual, Fourth Edition, Van Nostrand Reinhold (International) Co. Ltd., London (1988), particularly pages 282-591; with regard to paints: C. H. Hare, Protective Coatings, Technology Publishing Co., Pittsburg (1994), particularly pages 63-288. The foregoing references are hereby incorporated by reference herein for their teachings of ink, cosmetic, paint and plastic compositions, formulations and vehicles in which the compositions of this invention may be used including amounts of colorants. For example, the pigment may be used at a level of 10 to 15% in an offset lithographic ink, with the remainder being a vehicle containing gelled and ungelled hydrocarbon resins, alkyd resins, wax compounds and aliphatic solvent. The pigment may also be used, for example, at a level of 1 to 10% in an automotive paint formulation along with other pigments which may include titanium dioxide, acrylic latices, coalescing agents, water or solvents. The pigment may also be used, for example, at a level of 20 to 30% in a plastic color concentrate in polyethylene.